1. Field of the Invention
This invention relates in general to intermediate amplifiers for transmission lines of the coaxial type.
2. Description of the Prior Art
Remotely fed intermediate amplifiers are required in coaxial lines between input and output connections to the coaxial line and the outer conductor of the coaxial conductor of the input and output line are connected together and a supply voltage is supplied to the intermediate amplifier and a capacitor is arranged between the outer conductor of the coaxial lines and the intermediate amplifier. An intermediate amplifier of this type is described in German Auslegeschrift No. 1,940,517.
At amplifier points for wide band transmission systems, for example, carrier frequency line amplifiers, such amplifiers must be remotely fed in series with direct current through the inner conductors of the coaxial cables and the amplifier reference potentials are the direct current potential of the inner conductors of the coaxial cable and cannot, therefore, be directly connected to the outer conductors of the coaxial cable. Generally, a housing is provided to which the grounded outer conductors of the coaxial cables are connected. Between the amplifier reference potential and the housing, there must be a low ohmic alternating current connection in the frequency range of the transmission band of the coaxial cable which connection is made by means of a blocking capacitor.
FIG. 1 illustrates an arrangement of prior art remote amplifiers using a simple shunt circuit for a remotely fed intermediate amplifier 1 which receives direct current and is series fed. The direct current supply flows through the inner conductor of the coaxial cable 21 or alternatively through the coaxial cable 22 and passes around the remotely fed amplifier 1 through inductive chokes 51 and 52 as shown and is used to supply the amplifier feed voltage U. The junction point X between the chokes 51 and 52 is at zero potential with regards to AC voltage and is connected to the outer conductor Y of the coaxial cables 21 and 22 through the capacitor 3 for this type of remote feed. The capacitor 3 must have a high voltage electrical strength of, for example, several thousand volts since a substantial voltage differential can occur between the inner and outer conductors of the coaxial cable and this capacitor must not be destroyed if overvoltages occur such as a result from electrical storms and other factors.
To obtain the high voltage breakdown resistance of the capacitor produces a number of problems. For example, due to the high voltage breakdown strength it must be of a certain minimum size which causes self-inductance which is not negligible at high frequencies as well as this produces parallel resonance at certain frequencies. Alternating currents i.sub.A in the output circuit must flow through capacitor 3 and such current causes a voltage drop U.sub.C across the capacitor if the impedance of the capacitor 3 is not negligibly small. Such voltage in turn causes a current i.sub.E in the input circuit which comprises a feedback that takes place from the output to the input. This can cause instability or at least changes in the frequency response curve of the amplifier 1 which is very undesirable.
Therefore, because of the particular design of the capacitor 3, measures must be taken so as to achieve a sufficiently high cyclical attenuation or in other words a sufficiently small feedback between the output and the input. A capacitor especially designed for this purpose, has been described in German Auslegeschrift No. 1,940,517.
If for compensating the amplifier field attenuation, it is required that the amplifier 3 have an especially high amplification in a certain specific frequency range near the upper band limit, it can occur that the necessary cyclical attenuation can no longer be achieved even when using a special type capacitor. Also, it is not feasible to connect several identical or different capacitors in parallel since parallel resonances can occur in amplifier arrangements, if individual elements act inductively at certain frequencies but others are still capacitive and, thus, undesirable effects can occur.
It is also possible to provide a voltage transformer in the feed supply direct current path with the output of the transformer electrically separated from its input so that the reference potential of the intermediate frequency amplifier can be directly connected to the outer conductor of the coaxial cable and a capacitor is not required in such arrangement. See, for example, German Offenlegungsschrift No. 2,355,014. In such arrangement, a special current supply device is required for each intermediate amplifier. In addition to the lack of reliability and expense for the components of such arrangement, in addition a current supply of this type has the disadvantage that its efficiency is limited so that feed supply energy is lost and heat is generated.